Most hydrogen is produced as a mixture with carbon dioxide by the reaction of carbon monoxide with water, and this production reaction is known as the water gas shift reaction. This reaction is thermodynamically exothermic, but is carried out at a high temperature of 200 to 500° C. in order to ensure high reaction rates. Because it is an exothermic reaction, the equilibrium shifting to hydrogen at high temperatures is limited. For this reason, membrane reactors have been proposed which can separate produced hydrogen during the reaction. This is because the equilibrium can be shifted more toward production by removing only hydrogen through the reactor wall in equilibrium shift. Because the reaction is carried out at high temperatures as described above, it is essential to secure a technology for ensuring the high thermal stability of a membrane which serves as a wall in the membrane reactor.
A membrane formed of zeolitic imidazolate framework-(hereinafter referred to as “ZIF-8”) has a pore size of 0.34 nm and can separate hydrogen (0.289 nm) and carbon dioxide (0.33 nm) from each other by the size difference therebetween. The H2/CO2 separation ability of ZIF-8 membranes can be found in various literatures (R. Krishna and J. M. van Baten, J. Membr. Sci., 2010, 360, 323-333; Q. Liu et al., J. Am. Chem. Soc., 2013, 135, 17679-17682).
Among methods for synthesizing ZIF-8 membranes, counter diffusion is a method of synthesizing a membrane by controlling the diffusion rate and reaction rate of Zn2+ ions and 2-methylimidazole molecules, which constitute ZIF-8. The Zn2+ ions are inside a porous support, and the 2-methylimidazole molecules are outside the porous support. Thus, these ions and molecules tend to diffuse in opposite directions due to the difference in concentration therebetween. The two materials meet each other on the porous support surface to form ZIF-8 grains, thereby synthesizing a membrane. The ZIF-8 membrane synthesized as described above shows high separation coefficients in propylene/propane separation (H. T. Kwon and H. K. Jeong, J. Am. Chem. Soc., 2013, 135, 10763-10768).
Currently, ZIF membrane technology is being developed in the direction of suggesting new membrane synthesis methods. For actual application to the water gas shift reaction, the thermal stability of the membrane should be confirmed and secured through H2/CO2 measurement at high temperatures.
Accordingly, the present inventors have made extensive efforts in order to solve the above-described problems, and as a result, have found that when a membrane is produced inside a hierarchically structured porous support by the counter diffusion method. The resulting membrane will have an increased stability against physical changes caused by external impact or the like, and will not be exposed directly to high-temperature gases, unlike a conventional membrane that shows low thermal stability due to direct contact with high-temperature H2 and CO2 gases, and thus can ensure higher thermal stability than the conventional membrane, thereby completing the present invention.